1. Field of Invention
The present invention relates to a PWM sound device, and more particularly to energy saving PWM sound device behaves like an inductor to block most of the PWM frequency and its harmonics to the speaker such that the power consumption and EMI is significantly reduced.
2. Description of Related Arts
As the fast developing technology in electronics and semiconductors, electronics devices are work more efficient. Energy saving are the obvious trend during the development of almost every electronic product. Especially in fields such as voice-IC products, equipments must adapt higher audio quality and low energy consumption. The requirement of being better sound quality but environmental energy saving can never be stopped.
But there is a bottleneck for audio signal transportation. Electromagnetic interference (EMI) always reduces the sound quality of sound products. Traditionally voice-ICs contain PWM (Pulse-Width Modulation) direct drive output, which the speaker is connected to the PWM output. The output signal contains three parts: an audio signal, PWM carriers signal, and the aliasing noise (due to low sampling rate of audio signal), wherein this carriers frequency is normally few 10 kHz. Accordingly, since there is an inference between the audio signal and the electromagnetic force, the sound quality of the voice-ICs is dramatically reduced.
Moreover, in voice-IC products, power consumption is always an important concern. Traditionally such power consumption is measured by current drain of the power source (battery). Accordingly, normally an ammeter is connected in series of the power source. When the voice-IC is playing, such ammeter can measure the current drain of the voice-IC. But such measurement method has a big disadvantage. Most of the voice-IC outputs are under PWM format. When the voice-IC is under playing, current will be consumed only when the output is in high-level instant. No current will be consumed when the output is in low-level instant.
In voice-IC, the carrier frequency is normally from 20 kHz-30 kHz (30 us-50 us in period). Under such high frequency, the measured current consumption will be shown under high frequency frustration. No matter analog or digital ammeter is used, the reading will still under frustration and it is so difficult to get an accurate result. Moreover, PWM carriers signal carrier dissipation further leads to power waste. Accordingly, the audio signal up to 4 kHz is the mainly playback sound which is concerned. The signal of carrier frequency (which is not auditable) and the aliasing noise (unwanted “metallic” sound) are not useful, and even worse to the playback sound. Such signals also consume power as well.